Severe combined immunodeficiehcy disease due to inherited deficiency of adenosine deaminase (ADA) is a focal point for evaluation of innovative approaches to therapy by enzyme replacement with polyethylene glycol-modified bovine ADA (PEG-ADA) and by somatic cell gene supplementation. The research objectives of this proposal are aimed at understanding the genetic, biochemical and immunologic factors that limit or determine the response of patients to PEG-ADA, and to investigate a strategy that may permit wider application of PEG-enzyme therapy for other inborn errors of metabolism. Our specific aims are 1) To characterize mutations in ADA alleles of PEG-ADA patients and study the expression of these mutant alleles in vitro and in appropriate cells in order to identify those that allow sufficient residual ADA activity to permit immune reconstitution during enzyme replacement; 2) to investigate the basis for selective expression of certain mutant ADAs in T cells of some patients; and 3) to sequence the cDNA for bovine ADA in order to characterize the binding site of inhibitory antibodies to ADA that develop in patients undergoing treatment with PEG-ADA; and to attempt to 'neutralize' these epitopes with a combination of directed mutagenesis and PEG-modification.